Out of the 39 families or groups of Freshwater fishes, 9 only are represented in the Tropical American region. This may be accounted for by the fact that South America is too much isolated from the other regions of the Equatorial zone to have received recent additions to its fauna. On the other hand, the number of species exceeds that of every other region, even of the Indian, with which, in regard to the comparative development of families, the Neotropical region shows great analogy, as will be seen from the following Table:-- INDIAN. NEOTROPICAL. Siluridæ 200 sp. Siluridæ 276 sp. Cyprinidæ 330 sp. Characinidæ 226 sp. Labyrinthici 25 sp. Chromides 80 sp. Ophiocephalidæ 30 sp. Cyprinodontidæ 60 sp. Mastacembelidæ 10 sp. Gymnotidæ 20 sp. In both regions the great number of species is due to the development of numerous local forms of two families, the _Characinidæ_ taking in the New World the place of the _Cyprinidæ_ of the Old World. Thereto are added a few smaller families with a moderately large number of species, which, however, is only a fraction of that of the leading families, the remainder of the families being represented by a few species only. The number of genera within each of the two regions of the two principal families is also singularly alike; the Indian region having produced about 45 Siluroid and as many Cyprinoid genera, whilst the Neotropical region is tenanted by 54 Siluroid and 40 Characinoid genera. These points of similarity between the two regions cannot be accidental; they indicate that agreement in their physical and hydrographical features which in reality exists. Of Ganoids, we find in Tropical America one species only, _Lepidosiren paradoxa_, accompanied by two Osteoglossoids (_Osteoglossum bicirrhosum_ and _Arapaima gigas_). Autochthont and limited to this region are the _Polycentridæ_, all the non-African genera of _Chromides_ and _Characinidæ_; of Siluroids, the _Hypophthalmina_, _Aspredinina_, and _Stegophilina_, and the majority of _Pimelodina_, _Hypostomatina_, and _Doradina_; the herbivorous Cyprinodonts or _Limnophagæ_, and numerous insectivorous Cyprinodonts or _Carnivoræ_; and the _Gymnotidæ_ (Electric eel). The relations to the other regions are as follows:-- 1. The resemblances to the Indian and Tropical Pacific regions partly date from remote geological epochs, or are partly due to that similarity of physical conditions to which we have already referred. We have again to draw attention to the unexplained presence in South America of a representative of a truly Indian type (not found in Africa), viz. _Symbranchus marmoratus_. On the other hand, a direct genetic affinity exists between the Neotropical and African regions, as has been noticed in the description of the latter, a great part of their freshwater fauna consisting of descendants from a common stock.